As one technique of the mass spectrometry, a technique called the “tandem analysis” or “MSn analysis” is commonly known. The tandem analysis is an analytical technique including the following steps: an ion having a specific mass-to-charge ratio as the target is initially selected from various ions generated from the compounds in a sample; the selected ion (which is normally called the “precursor ion”) is fragmented by a collision-induced dissociation (CID) or similar dissociating operation; and a mass spectrometry for the ions generated by the fragmentation (which are normally called the “product ions”) is performed. In recent years, this technique has been widely used, mainly for the identification and structural analysis of substances having high molecular weights. For some compounds that cannot be broken into sufficiently small fragments by a single dissociating operation, the selection of the precursor ion and the dissociating operation for that precursor ion may be repeated a plurality of times (i.e. an MSn analysis with n being equal to or greater than three may be performed).
Examples of the commonly known mass spectrometers for tandem analysis include a triple quadrupole mass spectrometer having two quadrupole mass filters placed on the front and rear sides of a collision cell (which is also called the “tandem quadrupole mass spectrometer”) as well as a Q-TOF mass spectrometer using a time-of-flight mass analyzer in place of the rear quadrupole mass filter in the triple quadrupole mass spectrometer. These types of mass spectrometers can perform the selection and dissociation of the precursor ion only one time, and therefore, only a tandem analysis of up to MS2 (=MS/MS) analysis can be performed. By comparison, in the case of an ion trap mass spectrometer using an ion trap which is capable of repeatedly performing the selection and dissociation of the precursor ion a plurality of times, or an ion-trap time-of-flight mass spectrometer including an ion trap combined with a time-of-flight mass spectrometer, it is in principle possible to perform an MSn analysis with no limitation of the value of n (although n is practically limited to five or so for the sake of sensitivity).
The process of identifying a compound in a sample using such a tandem analysis is normally performed as follows: An ion having a specific mass-to-charge ratio originating from the compound is fragmented, and a mass spectrometry for the product ions generated by the fragmentation is performed to obtain an MS2 spectrum. The peak pattern of this measured MS2 spectrum is compared with those of the MS2 spectra of known compounds stored in a compound database, and the degree of similarity of the pattern is calculated. With reference to this degree of similarity, the kind of compound is determined. Therefore, for an exact identification of the compound, it is essential that the peak information observed in the mass spectrum (primarily, the mass-to-charge-ratio values) be highly accurate. In recent years, the performance of mass spectrometers has noticeably improved, and a peak which is merely observed as a single peak on a mass spectrum obtained with a conventional device can often be resolved into a plurality of peaks with a device having a high mass-resolving power. With such an improvement in the mass-resolving power and mass accuracy, the reliability of the compound identification by the previously described database search has also dramatically improved.
While the mass-resolving power of the device has improved in the previously described manner, it is difficult to extremely decrease the mass-to-charge-ratio width which is set for selecting the precursor ion. The reason for this is because the selection characteristics of the window for extracting an ion having a specific mass-to-Charge ratio (“mass window”) show a comparatively gradual change (i.e. the transmittance gradually decreases) at both end portions of the window, which means that narrowing the mass-to-charge-ratio selection width of the window decreases the amount of product ions to be subjected to the dissociating operation, making it difficult to detect the product ions with a sufficiently high level of sensitivity (for example, see Patent Literature 1). For such reasons, in commonly used mass spectrometers, the mass-to-charge-ratio selection width for the precursor ion is set at approximately 0.5-2 Da, Therefore, if there are a plurality of kinds of ions with a small difference in mass-to-charge ratio (e.g. 0.5 Da or smaller), a plurality of peaks of the product ions created by the dissociation of a plurality of different ion species will be mixed on the eventually obtained MS2 spectrum. If the peak information derived from such an MS2 spectrum is simply used in the database search, it will be difficult to identify the compound with a sufficiently high level of accuracy.